In a previously known percussion device, a percussive piston strikes against a drill steel over a drill shank, whereby is produced a shockwave which is essentially twice as long as the length of the percussive piston. The shockwave moves forwardly in the drill steel with the speed of sound in steel.
The drill string end and thereby the drill bit, which is attached to the string, moves forwardly a distance which depends on the length of the shockwave and the striking speed of the piston. In order to obtain rock crushing in front of the drill bit, the forward movement of the drill bit must be sufficiently great.
Besides, depending on the properties of the rock, certain types of rock require longer strike lengths of the drill bit than other types of rock in order to be effectively disintegrated.
The material in the percussion device and the drill steel, and in particular the steel strength, limits possible piston striking speed. In order to obtain sufficient drill bit displacement, the percussion device thus must be dimensioned such that the percussive piston has a relatively long axial length in order to ensure sufficiently long shockwave length, so that drilling can be effective in various types of rock.
The length of the percussive piston is also dimensioning for the total length of the drilling machine, which makes it a problem to provide drilling machines with smaller dimensions which are for example more suitable for use in more confined spaces.